This invention relates generally to imaging systems and more particularly to, systems and methods for determining a position of a support used within an imaging system.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an x-y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through a subject, such as a patient, being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the subject to be imaged so that the angle at which the x-ray beam intersects the subject constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the subject includes a set of views made at different gantry angles or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the subject. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
As CT scanners continue to use smaller detector element or cell sizes, a plurality of constraints applied to table positioning accuracy and stability become more stringent. In addition, reconstruction techniques using variable table speed during the CT scan make an accurate measurement of table position more critical.